Extreme ultraviolet light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma (“LPP”) the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam.
One particular LPP technique involves irradiating a target material droplet with one or more high energy pulses. In this regard, CO2 lasers may present certain advantages as a drive laser producing high energy pulses in an LPP process. This may be especially true for certain target materials such as molten tin droplets. For example, one advantage may include the ability to produce a relatively high conversion efficiency, for example, the ratio of output EUV in-band power to drive laser input power.
In more theoretical terms, LPP light sources generate EUV radiation by depositing laser energy into a source element, such as xenon (Xe), tin (Sn) or lithium (Li), creating a highly ionized plasma with electron temperatures of several 10's of eV. The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma in all directions. In one common arrangement, a near-normal-incidence mirror (often termed a “collector mirror”) is positioned at a distance from the plasma to collect, direct (and in some arrangements, focus) the light to an intermediate location, e.g., focal point. The collected light may then be relayed from the intermediate location to a set of scanner optics and ultimately to a wafer. In a typical setup, the EUV light must travel within the light source about 1-2 m from the plasma to the intermediate location, and as a consequence, it may be advantageous, in certain circumstances, to limit the atmosphere in the light source chamber to gases having relatively low absorptance of in-band EUV light.
For EUV light sources designed for use in high volume manufacturing (HVM) environments, e.g. exposing 100 wafers per hour or more, the lifetime of the collector mirror can be a critical parameter affecting efficiency, downtime, and ultimately, cost. During operation, debris are generated as a by-product of the plasma which can degrade the collector mirror surface. These debris can be in the form of high-energy ions, neutral atoms and clusters of target material. Of these three types of debris, the most hazardous for the collector mirror coating is typically the ion flux.
Generally, for the configuration described above, the amount of neutral atoms and clusters from the droplet target impinging onto the collector may be small since most of the target material moves in a direction pointing away from the collector surface, (i.e., in the direction of the laser beam). In the absence of debris mitigation and/or collector cleaning techniques, the deposition of target materials and contaminants, as well as sputtering of the collector multilayer coating and implantation of incident particles can reduce the reflectivity of the mirror substantially. In this regard, co-pending, co-owned U.S. application Ser. No. 11/786,145 filed on Apr. 10, 2007, entitled LASER PRODUCED PLASMA EUV LIGHT SOURCE, discloses a device in which a flowing buffer gas such as hydrogen at pressures at or above about 100 mTorr is used in the chamber to slow ions in the plasma to below about 30 eV before the ions reach the collector mirror, which is typically located about 15 cm from the plasma.
It is currently envisioned that about 100 W of EUV power, or more, will need to be delivered to a scanner/stepper to allow for efficient high volume EUV photolithography. To obtain this output power, a 5-20 kW drive laser, e.g. CO2 laser, may be used to irradiate a source material such as a stream of tin droplets. Of the 5-20 kW of power delivered within the EUV light source chamber, calculations indicate that about 20%-80% of this power may be transferred to a buffer gas in the chamber.